1. Field of the Invention
The present invention relates to a phase-monopulse radar apparatus.
2. Description of the Related Art
In a phase-monopulse radar apparatus, a wave reflected from a target is received using two receiving antennas 50, as shown in FIG. 1, and the azimuth angle θ of the target is determined from the phase difference φ between the two received signals by using, for example, the following equation.θ=sin−1(λφ/2πdo)  (1)where λ is the wavelength of the radar wave, and do is the antenna spacing.
An FM-CW radar transmits a wave frequency-modulated by a triangular wave, produces a beat signal by mixing the received wave with a portion of the transmitted wave, and obtains the distance and the relative velocity of the target from the sum and the difference between the frequencies of the beat signals produced during the rising portion and the falling portion, respectively, of the triangular wave; such an FM-CW radar is used as an automotive radar. In this type of FM-CW radar, the received-phase difference φ is calculated from the phase values of the peaks that appear in the Fourier transformed results of the beat signals.
However, in a situation where a plurality of peaks due to reflections from a plurality of targets are so close to each other that they cannot be resolved on the frequency axis, what would be observed as the received-phase difference φ would be the phase of the combined version of the reflected waves returned from the plurality of targets, and the correct target azimuth could not be determined. This condition will be referred to as the azimuth fault condition (or multi-target condition). Such a situation occurs, for example, in the case of an automotive FM-CW radar, when two vehicles are traveling ahead side by side at approximately the same speed in the lanes adjacent to the right and left of the lane along which the radar-equipped vehicle is traveling, or when two vehicles are traveling ahead side by side at approximately the same speed, one in the same lane as the radar-equipped vehicle and the other in an adjacent lane. In such cases, as the distance and the relative velocity are nearly the same for the two vehicles, the frequencies during the rising portion and the falling portion of the triangular wave are close to each other between the two vehicles and therefore cannot be resolved, the resulting situation being the azimuth fault condition.